The present invention relates generally to post assemblies installed along roadways and designed to carry various types of signs or related structures and, more particularly, to post assemblies that include breakaway systems that allow the assemblies to shear or break off upon impact, and, most particularly, to breakaway systems integrally incorporated into the post assemblies.
Man-made fixed objects frequently populate the sides of roadways including, for example, highway signs, roadway lighting, traffic signals, railroad warning devices, motorist-aid call boxes, mail and newspaper boxes, utility poles and other types of structures typically mounted on post assemblies. Federal, state, and many local governments require that sign post assemblies and other similar structures meet certain requirements set forth by the Federal Highway Administration (FHWA). For example, the American Association of State Highway Transportation Officials"" (AASHTO) Standard Specifications for Structured Supports for Highway Signs, Luminaires, and Traffic Signals (1994), which has been adopted by the FHWA, sets forth various requirements related to sign posts and breakaway systems. The Standard requires the maximum change in velocity for a standard vehicle (1800 pounds (816.5 kg)), or its equivalent, striking a sign post assembly at speeds from 20 mph to 60 mph (32 km/h to 97 km/h), not exceed 16.0 fps (4.87 mps), but preferably not exceed 10 fps (3.05 mps). In order to meet these requirements and to reduce the severity of injury to occupants and damage to vehicles when a vehicle impacts a sign post assembly, the assemblies often include a breakaway system. The breakaway systems are designed to allow the sign post assembly and/or its associated structure to break off or bend away when hit by a vehicle with a predetermined impact force and from a particular direction of travel. As of Oct. 1, 1998, the recommendations contained in the National Cooperative Highway Research Program (NCHRP) Report 350 concerning breakaway systems for sign post assemblies installed along highways became effective. These recommendations include the requirement that any sign post assembly must be capable of breaking away from any direction from which it is impacted. For all practical purposes, this requires a sign support system that will break away from all angles of impact (i.e., 360 degrees).
The design of any sign post assembly must balance the requirements that the assembly be capable of not only holding a sign without bending or breaking from wind load, but also be able to shear off in the event of a vehicular impact. One commonly used sign post design uses perforated square tubing. This tubing has consistent breakaway characteristics, but not at all impact angles. For example, the tubing will generally not break when impacted by a vehicle at one of the four corners of the tubing at appropriate speeds. Therefore, this and other forms of sign post assemblies typically are used in connection with a wide variety of add-on devices engineered to meet the breakaway standards.
These add-on devices include couplers, slip planes, plastic hinges, fracture elements, and various combinations of these. For example, one prior art device includes a sign post assembly having a lower ground post anchored into the ground and partially protruding therefrom. An elongated upper support post designed to carry a sign is then attached to the protruding portion of the lower ground post using a coupler or fastener, manufactured separate and apart from the support post (and possibly using a different type of material), having a breakaway characteristic, such as bolts which are designed to fail under predetermined tensile stresses.
Slip base type mechanisms activate when two parallel plates slide apart as bolts are pushed out under impact. Typically, the parallel plates are inclined with respect to the roadway or ground to ensure that the support post will move upward to allow the impacting vehicle to pass under the sign without its hitting the windshield or the top of the car. One significant disadvantage with slip base type breakaway supports is that the support post will only break away from a limited impact angle. That is, the inclined slip base can only be struck from one direction to yield satisfactorily and thus will not meet the new requirements for signs placed in medians, traffic islands or other locations in which it might be impacted from any of several directions. An improved slip base mechanism is triangular-shaped and is designed to release when struck from any direction. However, these types of sign supports are difficult to install and maintain and are subject to problems caused by wind that causes the bolts in the slip base to loosen. Also, the clamping bolts used with slip base mechanisms must be properly tightened (and maintained). If the bolts are over-torqued, the higher friction between the slip plates may prevent the post from releasing when impacted. Thus, a breakaway system that is not dependent on a specific torque requirement would be beneficial.
These prior art techniques suffer from significant disadvantages in addition to those previously mentioned. For example, they all require the use of an auxiliary add-on device to meet the breakaway criteria. These auxiliary devices increase the cost of the sign post assembly and typically make the installation, maintenance, repair, and replacement of damaged parts due to collisions a difficult and expensive operation.
Accordingly, there is a continuing need for an improved sign post assembly capable of meeting the new breakaway standards. The improved sign post assembly preferably would include a breakaway system incorporated integrally into the support post and not require the use of any type of add-on device. Desirably, the support post having the integrated breakaway system would be structurally adequate to support the sign or other device mounted thereon and would sufficiently resist ice and wind loads while being stable over a suitable time period of use.
The present invention is directed to an intentional modification of an as-rolled support post to form a defined integral weakened region which will break away upon impact by a vehicle. As used herein, the phrase weakened region refers to an area intentionally designed or altered to fracture upon impact. The weakened region may be formed in any number of ways including swaging (i.e., reducing the cross-sectional size of a portion of the support post), scoring, grooving, dimpling, perforating, or other forms of deforming, re-sizing, or reshaping the support post. The integral weakened region may be included in any form of support post including any cold rolled post shape used in a support method including, but not limited to, traffic signs, delineators, luminaire supports, mailbox and newspaper box supports, motorist-aid call box supports, work-zone temporary signs, barricades, snow fences, and other similar forms of support. For example, the support post may be tubular (round, square, hexagonal, octagonal, etc.), angular, channeled, or flanged channels having a cross-sectional shape similar to a hat. The support post integrally incorporates a weakened region that not only allows for uniform breakability from any approach angle, but also designates a predesigned shear point.
A preferred embodiment of the present invention is directed to a breakaway support post of a selected shape and having one end adapted for carrying an associated structure, such as a traffic sign. The support post includes an integral weakened region providing an area of structural weakness for allowing the support post to shear upon impact by a vehicle traveling at a minimum speed and impacting the support post from any impact angle. The breakaway support post may be anchored directly into the terrain surrounding the post or it may be attached to a ground post anchored below (or inserted into), and partially protruding above, the surrounding terrain. Preferably, the support post is installed such that the integral weakened region is at a predetermined height above the terrain, for example, approximately four inches above the surrounding terrain, so that the portion of the support post which remains anchored to the terrain after impact by a vehicle will not snag on any part of the undercarriage of the vehicle. Preferably, the cross-sectional area of the integral weakened region is the same or slightly less than the cross-sectional area of the other portion(s) of the support post. Thus, the support post having the integral weakened region retains its wind-loading characteristics while having a defined portion capable of breaking away in a consistent manner. In a preferred embodiment, the integral weakened region includes a groove circumscribing the weakened region to further increase and assure the breakability of the support post in a particular area. While the present invention may be used to form a breakaway support post having any selected shape, an illustrated shape is a square tube wherein the integral weakened region is substantially circular in cross-sectional shape.
Another embodiment of the present invention is directed to a sign post assembly that includes a tubular ground post mounted into surrounding terrain, a support post adapted to carry a traffic sign (or similar structure), and a hollow, tubular coupling for connecting the support post to the ground post. Preferably, the coupling attaches the support post to the ground post and includes an integral weakened region providing an area of structural weakness for allowing the coupling to shear upon impact of the sign post assembly by a vehicle traveling at a minimum speed and impacting the sign post assembly from any impact angle. The coupling may be of smaller cross-sectional size than both the support and ground posts such that the ground post and the support post may be slideably received onto the tubular coupling, or the coupling may have a larger cross-sectional size than the support and ground posts such that the posts slide into the coupling. Preferably, the support post, ground post and initial coupling are square in cross-sectional shape and the integral weakened region of the coupling is substantially circular in cross-sectional shape.
Yet another embodiment of the present invention is directed to a hollow, tubular coupling for attaching a support post to a ground post, the coupling having a first end adapted to be attached to the support post, a second end adapted to be attached to the ground post, and a breakaway region integrally formed between the first end and the second end to provide an area of structural weakness capable of breaking as the coupling is subjected to a predetermined lateral load from an object impacting the coupling at any impact angle.
The present invention further provides a method of producing a breakaway support post capable of breaking away upon impact from any impact angle. The method includes the steps of creating an integral weakened region in a support post having a selected shape to provide an area of structural weakness to allow the support post to shear upon impact by a vehicle traveling at a given speed and impacting the support post from any impact angle. For example, the support post may be formed from a hollow, square tube and the method includes using a hydraulic or mechanical press or a manual swaging tool to swage or deform a region of the support post to create an integral weakened region in the support post substantially circular in cross-sectional shape.